This invention relates to devices for protecting RF instruments from damage due to injection of high power surges. More particularly, this invention relates to a coaxial relay adjoining an RF instrument connector for protecting an RF instrument from reverse power surges.
RF instruments such as signal generators, spectrum analyzers, network analyzers and measuring receivers may be exposed to high power surges of 50 Watts or more inadvertently injected into an external signal port. Such instruments have sensitive internal circuitry which would be damaged upon exposure to such reverse power surges. Accordingly, there has been a need to protect the internal circuitry, isolating the circuits from reverse power surges.
Typically, limiting diodes are placed in-line to an output connector enabling output current to flow, while clamping the voltage level on any injected reverse power surge signals. In practice, however, the size of the diodes is limited so as to maintain matched impedance along the signal path. It is well known that impedance matching is needed along RF instrument signal paths to minimize signal reflection and resulting degradations over the operating frequency range. As a result, the limited diode size enables the diodes to protect against high power surges for only a short period of time. Thereafter the diodes fail and no longer serve to limit the voltage.
To adequately protect the internal circuitry of an RF instrument, a relay is used between the connector and the limiting diodes. Normally, the relay is closed allowing signals to flow in either direction. Thus, an output signal can flow in one direction and an injected signal can flow back along the signal path into the instrument in the opposite direction. In response to a reverse power injection above a specified threshold, the relay is triggered open. Creating the open circuit saves the limiting diodes and internal circuitry from damage.
The limiting diodes provide an interim time period for protecting the internal circuitry while the reverse power surge is detected and the relay is switched open. By switching the relay open, the internal circuitry is isolated from the injected power surge. Like the diodes, the relay also must provide good impedance matching capabilities to avoid signal reflection and related signal degradation during normal operation.
One type of relay used for reverse power protection is a coaxial relay. U.S. Pat. No. 4,870,385 (Jewell) discloses a coaxial relay switch for RF signals. An embodiment of the switch 10 as shown in FIG. 1 includes an electrically insulating body 12 having a central channel receiving a reed switch 14. Reed switches are used for applications where RF signals are to be switched on or off in a very short time period. A reed switch typically includes ferromagnetic contacts hermetically sealed in a glass vial. In the presence of a specified magnetic field the contacts are drawn together closing the signal path. In the absence of the magnetic field the contacts are relaxed out of physical communication opening the signal path. The reed switch contacts extend out of the glass enclosure within the channel 13 of body 12. A portion of the exposed contact is surrounded by an insulator 16 which abuts the glass enclosure. Toward a distal end of the contact, a center contact receives the reed switch contact. The center contact establishes coupling to an RF connector 18.
To achieve impedance matching in the embodiment described in U.S. Pat. No. 4,870,385, an electrically conductive resin is injected into the body 12 in the space between the body 12 and the glass enclosure 14, and in the space between the body 12 and a portion of the insulators 16. The resin is an electrically conductive epoxy resin, which upon curing, becomes mechanically rigid integrating the switch 14, insulators 16 and RF connectors 18. The cured resin provides a conformal outer conductor surrounding the switch assembly so as to eliminate impedance mismatching along the length of the glass enclosure. A problem with such a relay, and in particular with the use of a resin, is the typically low yield during manufacturing, the susceptibility to RF radiation, and the fragility of the finished assembly.
In creating a conductive epoxy resin, conductive material is integrated with the resin and frozen to a very low temperature for storage. Special chemicals for maintaining the sub-zero temperatures and special transportation to the parts manufacturer are required. The parts manufacturer receives the resin in the frozen state, thaws it, injects it into relay during assembly, then cures it into a rigid state. Once thawed, the resin typically has only a 1-2 hour shelf life within which it must be injected and cured. Curing is done by baking the component. In practice, there are many manufacturing constraints when using the resin which result in yields as low as 10%. In addition, the special handling of the resin causes the price to be relatively high. Low yields result in wasted material and extra cost. Accordingly there is a need for a relay having improved manufacturing yield characteristics.
Another problem with the resin is the difficulty in achieving a highly conductive state. While being conductive, the conductivity is not always high enough to avoid susceptibility to induced current on the center conductor in the presence of surrounding high frequency RF radiation. In practice, maintaining 70 dBc isolation or more has been difficult. To reduce susceptibility it is desirable to have an improved relay.